Re: [PATCH v3 7/8] dma-buf: transfer memcg charges via DMA_BUF_IOCTL_XFER_CHARGE

Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
- [High] dma-buf: unprivileged cgroup escape and OOM via dma_buf_xfer_charge()
- [High] dma-buf: race in mem_cgroup_transfer_dmabuf_charge() double-charges memcg
- [Medium] dma-buf: broken disabled-memcg handling in dma_buf_xfer_charge()
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commit 1660729e62b15fd915c69f26225530bd5ae30ce0
Author: Albert Esteve [off-list ref]

dma-buf: transfer memcg charges via DMA_BUF_IOCTL_XFER_CHARGE

This commit adds DMA_BUF_IOCTL_XFER_CHARGE to allow transferring a dma-buf's
memory cgroup charge to a cgroup identified by a cgroupfd or pidfd after
allocation. It also introduces a memcg_account flag in dma_buf_export_info for
exporters to opt in to memcg accounting, enabling it for the system heap.

[Severity: Medium]
When CONFIG_MEMCG is disabled, should dma_buf_xfer_charge() still validate
arg.flags and dmabuf->memcg_account, or return -EOPNOTSUPP?

Returning 0 before checking arg.flags != 0 and !dmabuf->memcg_account causes
calls with invalid flags, non-accounting exporters, or invalid file
descriptors to silently succeed under CONFIG_MEMCG=n, while also leaving the
!CONFIG_MEMCG stubs for mem_cgroup_get_from_fd() (which returns
ERR_PTR(-EOPNOTSUPP)) and mem_cgroup_transfer_dmabuf_charge() in
include/linux/memcontrol.h unreachable.

Also, when CONFIG_MEMCG=y and cgroup_disable=memory is passed on the kernel
command line (so mem_cgroup_disabled() is true at runtime),
mem_cgroup_get_from_fd() behaves inconsistently across its two branches:

mm/memcontrol.c:mem_cgroup_get_from_fd() {
	cgrp = cgroup_get_from_fd(fd);
	if (!IS_ERR(cgrp)) {
		rcu_read_lock();
		css = cgroup_css(cgrp, &memory_cgrp_subsys);
		if (!css || !css_tryget_online(css)) {
			rcu_read_unlock();
			cgroup_put(cgrp);
			return ERR_PTR(-ENOENT);
		}
...
	memcg = get_mem_cgroup_from_mm(mm);
	mmput(mm);
	return memcg;
}

For a cgroupfd, cgroup_css() is NULL and it returns ERR_PTR(-ENOENT), whereas
for a pidfd, get_mem_cgroup_from_mm() returns NULL rather than an ERR_PTR, so
dma_buf_xfer_charge() returns 0 for pidfds and -ENOENT for cgroupfds.

[Severity: High]
Can an unprivileged holder of a dma-buf file descriptor use
dma_buf_xfer_charge() to escape its own memory cgroup limit or trigger OOM
kills in another cgroup?

In the call chain:

  dma_buf_ioctl()
    -> dma_buf_xfer_charge()
      -> mem_cgroup_get_from_fd()
      -> mem_cgroup_transfer_dmabuf_charge()
        -> mem_cgroup_reserve_dmabuf(new_memcg, GFP_KERNEL, nr_pages)
          -> try_charge()

neither dma_buf_xfer_charge() nor mem_cgroup_get_from_fd() checks DAC or
cgroup delegation/write permissions (such as inode_permission() with MAY_WRITE
or cgroup_procs_write_permission()), capabilities, or ptrace_may_access() on
either the source cgroup (dmabuf->memcg) being uncharged or the target
cgroup/task (new_memcg) being charged.

Because /sys/fs/cgroup directories are normally world-readable (mode 0755, so
any user can open them with O_RDONLY | O_DIRECTORY) and pidfd_open() can open
a pidfd for PID 1 or any visible process without privilege, an unprivileged
process can resolve a handle to the root cgroup or to another cgroup on the
system.

When SELinux is not active, security_dma_buf_charge_cgroup() and
security_dma_buf_charge_task() return 0. Even with SELinux enabled,
selinux_dma_buf_charge_cgroup() returns 0 when the cgroup kernfs node has no
explicit security.selinux xattr set:

security/selinux/hooks.c:selinux_dma_buf_charge_cgroup() {
	rc = kernfs_xattr_get(cgrp->kn, XATTR_NAME_SELINUX, NULL, 0);
	if (rc == -ENODATA)
		return 0;
}

and neither hook checks permission to uncharge the existing dmabuf->memcg. As
a result, a confined client receiving a pre-charged dma-buf fd can move the
charge to the root cgroup to bypass its own memory.max limit, or move the
charge into a victim cgroup where mem_cgroup_reserve_dmabuf() with GFP_KERNEL
triggers direct reclaim and the memcg OOM killer.

[Severity: High]
What happens if two threads concurrently call DMA_BUF_IOCTL_XFER_CHARGE on the
same dma-buf?

In dma_buf_xfer_charge(), dmabuf->memcg == memcg is checked without
synchronization before calling mem_cgroup_transfer_dmabuf_charge(), which then
re-reads old_memcg = READ_ONCE(dmabuf->memcg) without checking whether
old_memcg == new_memcg.

If thread 1 transfers the buffer to new_memcg after thread 2 passes the
dmabuf->memcg == memcg check in dma_buf_xfer_charge() (or if both threads
concurrently transfer the buffer to the same new_memcg), thread 2 sees
old_memcg == new_memcg and still calls mem_cgroup_reserve_dmabuf(new_memcg,
GFP_KERNEL, nr_pages) before uncharging old_memcg. That transiently charges
2 * nr_pages against new_memcg for the same buffer, which can trigger reclaim,
memcg OOM kills, or -ENOMEM in new_memcg even though the buffer is already
charged to it.

In addition, old_memcg is read before mem_cgroup_reserve_dmabuf(new_memcg,
GFP_KERNEL, nr_pages), which can sleep in direct reclaim. If another thread
updates dmabuf->memcg while mem_cgroup_reserve_dmabuf() is sleeping, the
cmpxchg(&dmabuf->memcg, old_memcg, new_memcg) fails without retrying against
the updated old_memcg and returns false, which dma_buf_xfer_charge() reports
to userspace as -ENOMEM even though charging new_memcg succeeded.

Finally, if mem_cgroup_transfer_dmabuf_charge() is called when
mem_cgroup_disabled() is true, returning true at the top without storing
new_memcg in dmabuf->memcg or dropping new_memcg's reference would leak the
 caller's reference to new_memcg, since callers only call mem_cgroup_put() when
this function returns false.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261027-v2_20230123_tjmercier_google_com-v3-0-78d2c4cb8647@redhat.com?part=7